OK but then, why is it called "soft"? It shouldn't make the shutter release any softer, should it? What usefulness can it provide in real use?

Marketing.

I got one with one of my bodies, found it awkward and prone to snagging on things, so don't use it. There's an analogous accessory for guns called a "trigger shoe".

If you like them, if they make the shutter button feel better to you, then they're a good thing. I do use a trigger shoe on one of my guns, a rebarreled Mauser with a very narrow trigger - it gives a better feel for when it breaks.